A Review of Globally Important Ecosystems and Their Biodiversity of the Lower Mekong Basin
This annex comprises two sections. The first describes the wetland habitats of the Lower Mekong Basin and summarizes information on the biodiversity values of each. The second section summarizes the status of knowledge of the biodiversity of the Lower Mekong Basin and highlights its global significance. The definition of wetlands used for this annex is the broad definition adopted by the Ramsar Convention.
The Lower Mekong Basin is part of the Indo-Malayan Realm that extends from Pakistan to Papua New Guinea. It reaches into the Indo-Chinese and Sundaic Sub-regions and the Southwest China Unit although the Sundaic accounts for only a minute proportion of its total land area. The Lower Mekong Basin occupies a roughly central position in this Realm.
Knowledge of the status of the biodiversity of the Lower Mekong Basin is patchy. Recent efforts of both national and international agencies have often focussed on the larger and more exotic groups, e.g. mammals and birds, while ignoring smaller less obvious groups. For example, estimates of the total number of fish species in the Lower Mekong Basin range between 436 to 1,300 species. Also, while there is some information available on turtles, almost no information exists on groups such as frogs. Invertebrates remain largely undescribed.
The wetland biodiversity of the Lower Mekong Basin is severely threatened with some species on the brink of extinction. The destruction of natural habitats and illegal hunting for the wildlife trade are two critical factors. The root causes of the threats are discussed in Annex VII and details of the threats faced at each of the demonstration sites are given in Annex VI.
Wetlands are complex systems with a variety of physical, hydrological and vegetative characteristics. Simple categorisation of these systems is difficult. The structure and function of wetlands in the Lower Mekong Basin are inextricably linked to the seasonal hydrological pattern of the Mekong River, involving a wet season flow up to 10 metres higher than the dry season. For the purposes of this annex, the wetlands of the Lower Mekong Basin will be considered as the following descriptive units:
Upland tributaries and related systems including streams, reservoirs and headwaters.
Lowland river channels of the Mekong and its larger low-gradient tributaries
Permanently and seasonally inundated wetlands associated with seasonal rainfall and the annual inundation of the Lower Mekong Basin.
The Mekong Delta from south-eastern Cambodia through to the estuarine associations in the delta in Vietnam.
Tributaries on the east side of the Mekong flow from high in the Annamite Mountains and the Northern Plateau of Laos. Passing through dense stands of fringing Dipterocarpus sp. and evergreen gallery forest, these tributaries plunge steeply towards the Mekong Valley. The Korat Plateau of Thailand on the west side of the Mekong forms the southerly catchment of the middle reaches of the lower basin. Caves and waterfalls are common in karst areas. These areas have a high level of fish endemism and are vitally important for wetland associated mammal species.
The headwaters of the higher mountains are found on the eastern side of the Mekong River. These areas are characterised by the steepest gradients, higher altitude and rocky substrates. Smaller streams flow through a closed canopy forest, which opens as the streams widen.
Most fish species of the headwaters appear to be permanent residents, but, with few surveys to date, little is known of the overall fish diversity. Each new headwater survey identifies new species, and further survey efforts will certainly identify additional species and contribute to the understanding of fish diversity.
The Globally Near-Threatened Grey-headed Fish Eagle Ichthyophaga ichthyaetus inhabits the larger rivers, while the Brown Fish Owl Ketupa zeylonensis and Buffy Fish Owl Ketupa ketupu associate with smaller rivers.
The middle reaches of the medium-sized and smaller tributaries of the Mekong River contain diverse geological features that form a number of different habitats including deep-water pools, rapids, waterfalls and gorges. There is limited flooding in the wet season, and usually there is no floodplain.
The fish biodiversity of the middle reaches of most of the larger tributaries of the Mekong is not well known. However, the fish diversity of Nam Thun, a relatively large Mekong tributary in central Laos, has been studied in more detail than many other systems and will be used to demonstrate the fish diversity of these larger tributaries. In Nam Thun, there is an endemic fish, Luciocyprinus striolatus, which can grow to 60 kg, and according to local people is known to eat monkeys. Although found in other tributaries, it is not known from mainstream channels. A herbivorous carp of the genus Poropuntius (Poropuntius cf. deauratus) is the most abundant medium-sized fish species in the Nam Thun Basin and is possibly endemic. Two new endemic species, Onychostoma sp. nov. and Scaphognathops sp. nov., both occur in the middle reaches of the river. Of the three species of Tor spp. in the Mekong, two are endemic and one is known only from the Nam Thun basin. The eel Anguilla marmorata migrates from the Nam Thun to breed in the sea over 1,500km away. In total, of the 87 species of fish identified from the middle and upper reaches of this river, 33 (38%) were previously undescribed, and possibly endemic, species.
It is anticipated that surveys of other Mekong tributaries will yield similar levels of fish diversity. Many fish species of the middle reaches are not found in the headwaters of tributaries or mainstream channels of the Mekong or its larger tributaries. Of the 456 species identified in taxonomic studies of the Lower Mekong Basin by Kotellat (Kotellat 2000) 53 species (11%) are known from only single sub-basins.
Large complex rapids occur frequently on the main Mekong River and its tributaries. These support a diverse assemblage of fish (and invertebrate) species. Specialists to this habitat have a number of diverse adaptations to avoid being swept away including suckers, flattened bodies and expanded fins. The diversity of fish species from these rapids is known from only a few survey sites, and it is anticipated that further studies will reveal new species. Due to the comparatively small area of rapids in the Lower Mekong Basin, fish biodiversity experts recommend that every species permanently associated with rapids be classified as critically endangered1.
Dams and artificial reservoirs
The number of man-made dams and reservoirs in the Lower Mekong Basin is increasing with the development of new hydro-electric and irrigation schemes. Often these impoundments have limited seasonal variation in water height and possess a uniform underwater landscape.
In terms of freshwater biodiversity, reservoirs force a shift from a riverine system to a standing water community in which only a few species are able to adapt. The fish fauna of these reservoirs is usually stocked, often with exotic species. There has been little investigation of opportunities for aquaculture of native fish species in these reservoirs.
Mitigation program for dams often propose surrounding catchments as conservation areas. However, this does little to protect aquatic biodiversity, but assists in the management of the catchment for the operation of the facility (i.e. reduction of erosion and siltation into the reservoir). Concerns for catchment biodiversity conservation typically focus on large mammals, birds and forests.
Aquifers, caves and hot springs are common in limestone areas of the Lower Mekong Basin. The Khammouan karst in Laos is the most extensive karstic system in the Lower Mekong Basin. Extensive karst and cave systems also occur in western Thailand and Vietnam, few systems are found in Cambodia.
An endemic genus and species of blind cavefish Troglocyclocheilus khammouanensis and an endemic genus and species of cave crab have been identified from the Khammouan karst in Laos. There are also reports of un-pigmented cavefish in the Ke Bang karst in Thailand. As the majority of these areas have not been surveyed fully, it is certain that additional species of fish and invertebrates remain to be discovered.